Charlie Rowley, 45, and Dawn Sturgess, 44, fell ill in Amesbury, Wiltshire on June 30, just eight miles (13km) from Salisbury where Sergei Skripal and his daughter Yulia were poisoned.

The event was escalated to a “major incident” on July 4 after police confirmed the pair had been exposed to an “unknown substance.” Metropolitan Police Assistant Commissioner for Counter-Terrorism, Neil Basu, said there had been a “significant development” after officers declared a “major incident” earlier in the day.

Scotland Yard has since confirmed that the couple were exposed to Novichok following tests carried out by the Defence Science and Technology Laboratory at Porton Down.